The present invention relates to a two component infrared reflecting film, and more particularly to two component films which reflect light in the infrared region of the spectrum while suppressing second, third and fourth order reflections in the visible region of the spectrum.
Coextruded multilayer films have been made which comprise multiple alternating layers of two polymers with individual layer thicknesses of 100 nanometers or less. Such multilayer films are described, for example, in Alfrey et al, U.S. Pat. No. 3,711,176. When polymers are selected to have a sufficient mismatch in refractive indices, these multilayer films cause constructive interference of light. This results in the film transmitting certain wavelengths of light through the film while reflecting other wavelengths. The multilayer films can be fabricated from relatively inexpensive and commercially available polymer resins having the desired refractive index differences. The films have the further advantage in that they may be shaped or formed into other objects.
The reflection and transmission spectra for a particular two-component film are primarily dependent on the optical thickness of the individual layers, where optical thickness is defined as the product of the actual thickness of the layer times its refractive index. The intensity of light reflected from such films is a function of the number of layers and the differences in refractive indices of the polymers. Mathematically, the wavelength of the dominant, first order wavelength for reflected light (at normal incidence) is: ##EQU1## where .lambda..sub.I is the first order wavelength, n is the refractive index of the polymer, and d is the layer thickness of the polymer, and k is the number of polymer components. Films can be designed to reflect infrared, visible, or ultraviolet wavelengths of light depending on the optical thickness of the layers. When designed to reflect infrared wavelengths of light, such prior art films also exhibit higher order appearance for the films. Mathematically, higher order reflections will appear at ##EQU2## where m is the order of the reflection (e.g. 2, 3, 4, etc.) As can be seen, higher order reflections appear at fractions of the first order reflection. The films produced in accordance with the above mentioned Alfrey patent exhibit iridescence and changing colors as the angle of incident light on the film is changed.
For some applications, while reflection of infrared wavelengths is desirable, higher order reflections of visible light are not. For example, infrared reflecting films can be laminated to glass in buildings and automobiles to reduce air conditioning loads. The films may also be laminated to other substantially transparent plastic materials to reflect infrared wavelengths. However, the films must be substantially transparent to visible light so that the vision of those looking through the glass or plastic is not impaired.
It is possible to suppress some higher order reflections (i.e., reduce their intensity) by proper selection of the ratio of optical thicknesses in two component multilayer films. See, Radford et al, "Reflectivity of Iridescent Coextruded Multilayered Plastic Films", Polymer Engineering and Science, vol. 13, No. 3, May 1973. This ratio of optical thicknesses is termed "f-ratio", where f =n.sub.1 d.sub.1 /(n.sub.1 d.sub.1 +n.sub.2 d.sub.2). However, such two component films do not suppress successive second, third and fourth order visible wavelengths.
Other workers have designed optical coatings comprising layers of three or more materials which are able to suppress certain higher order reflections. For example, Thelen, U.S. Pat. No. 3,247,392, describes an optical coating used as a band pass filter reflecting in the infrared and ultraviolet regions of the spectrum. The coating is taught to suppress second and third order reflectance bands. However, the materials used in the fabrication of the coating are metal oxide and halide dielectric materials which must be deposited in separate steps using expensive vacuum deposition techniques. Also, once deposited, the coatings and the substrates to which they are adhered cannot be further shaped or formed. Further, the coatings are subject to chipping, scratching, and/or corrosion and must be protected. Finally, because vacuum deposition techniques must be used, it is both expensive and difficult to fabricate coatings which cover large surface areas.
Rock, U.S. Pat. No. 3,432,225, teaches a two component, four layer antireflection coating which utilizes specified thicknesses of the first two layers of the coating to synthesize an equivalent layer having an effective index of refraction which is intermediate that of the first two layers. However, Rock also uses metal halides, oxides, sulfides, and selenides which must be deposited in separate processing steps using vacuum deposition techniques.
Another technique has been suggested for a three-layer film comprised of two components which is equivalent in refractive index and optical thickness to a film comprised of three components. The third component is eliminated by synthesizing a three layer structure which has the same optical performance as a three component structure. See Ohmer, "Design of three-layer equivalent films", Journal of the Optical Society of AmericaVol. 68 (I), 137 (January 1978). However, Ohmer also uses vacuum deposition of metal oxides, halides, and selenides. Further, such a structure does not provide sufficient suppression of the fourth order reflectance band, thus hindering its optical performance.
Rancourt et al, U.S. Pat. No. 4,229,066 teaches a visible light transmitting, infrared reflecting multilayer coating utilizing metal halide sulfides and selenides. The materials have either a high or low index of refraction and must be deposited in separate steps using vacuum deposition techniques. In addition, Rancourt requires 10 layers in the repeat unit. Further, the coatings of Rancourt et al cannot be further shaped or formed after deposition.
Schrenk et al, U.S. Pat. No. 5,103,337, describes an all polymeric three-component optical interference film formed by coextrusion techniques which reflects infrared light while suppressing second, third and fourth order reflections in the visible region of the spectrum. However, the polymers in the film are required to have closely defined refractive indexes, which limits the choice of polymers which may be used. In addition, the production of the film requires separate extruders for each of the polymeric components.
Accordingly, the need still exists in this art for a two-component film which reflects infrared light, successfully suppresses multiple successive higher order reflections to prevent unwanted reflections in the visible range, allows a wide choice of polymers, and does not require complicated extrusion equipment.